Your data structure is good. What you need is a couple more fields in the join table to make this work.
You'll need an elapsed time field if you want to see the time spent on each position. That'll be End Time - Start Time. You also will need a summary field, maybe ElapsedTimeSum, that calculates the total of the elapsed time.
Then, based your chart on the join table, found set, summarized groups of records. Sort the join table on Position. Make your Slice Data equal to the summary field. That should do you.
I didn't need to use a join table after all but the key was in the creation of an extra field to summarize the Position count and then to sort the records appropriately.
As the portal is prefilled with 20 minute slots i do not need a counter for the minutes I can just multiply each occurence by 20minutes if needed.
Thanks for your fast reply.